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I like to poke at rusty heat exhangers with a screw driver in a few spots, if it is copmpromised you will usually find out much quicker than taking it out. If I don't put a hole in it and I still am uncertain I will explain to the customer and let them decide if they want to pay me to take it apart and stress the safety factor.

I do that too, but some go to far.

I was on a second opinion call for a hole in a HX. Natural draft furnace. I looked inside and saw the hole through rusty but thick metal. Someone took a screwdriver and I guess a hammer and beat a hole in it. The customer said they heard the hammering.
I left it up to them and advised them on how to proceed.
The most difficult part to prove it is not actually seeing them do it, but to assume they did by the noise.

I've also seen the low speed wire cut to the motor so they would have to call them back to check the heat- as well as the white LV wire cut, thermocouples cut and so on.

Well, an experienced tech can go on gut instinct because ive see it time and time again. Usually can tell right off if its just surface rust or rusted all the way through. Just get a Trane with a 20yr warranty on the Heat x and you will never have to worry.

I looked at a furnace this afternoon a goettl, yes that is spelled correctly, ok it is a spark ignition pilot with draft fan and the pilot is not getting pulled into combustion chamber and that chambers flame is not as clean burning as the others. My snake camera broke this summer but who thinks it is a cracked heat x

I understand what you mean about gut feelings on whether a furnace is safe or not but if you go by gut feelings and not evidence or CO readings than your relying on speculation and fear that you might have instilled to the unknowing homeowner. Just my opinion .

I understand this and it is why I started this thread. I do not want to put fear into my customers. I just want to do my job to the best of my abilities, at the same time keep my customers safe even if they do not want me to.

I have been on a couple of 2nd opinions already this fall for cracks. One was and one very definitely wasn't. Same company different techs. Seems to be the thing again this year.

I like to sell equipment too, but I will only do it honestly though.

This is why it's so important to find a good and honest company and stick with them. Don't get drawn in by any super cheap offers you hear in the media. The only way to really save on this stuff is to get good equipment from a good company that will install it correctly and then take care of it.

the gama/aga standard for inspecting hxs does include 50% loss of the metal as a failure, not too sure how you'd test that.

I check every furnace before repairing, since whoever touched it last will be blamed for any injuries, dust, "new" noises, the water heater breaking, the stove not lighting right, etc.

Some of Trane's gas packs are negative pressure gas valve/positive pressure hx design. the gas is injected into the intake of the combustion blower and blown through a screen (I think) into the comb chamber. -.02" wc on the gas valve.

I don't use a screwdriver for the "you did it" reason. WD40 sprayed on one side will bleed through any holes in rust spots.

Combustion analyzer for me. There's not a good way to see the entire heat exchanger no matter how you do it (unless you rip the furnace out). Also, I would not have a gas burning appliance without having a CO detector in the home either. And I always stress that rust isn't the only thing to crack a heat exchanger. Everybody has seen an overheated hx that has a crack in it or the awesome design of janitrol that uses the big rings (rivet type rings) that pop out and cause an opening in the hx.

It's just rocket science. It's not like it's heat and air work or something.

The most common thing that will cause a crack is lack of airflow.
Also Afa Co detectors, they will not sound until they detect... I believe 400ppm.
Also just because you may not pick up any Co means absolutely nothing.
The tiniest most insignificant thing like the ammount of c rap on the burner oriface
Can have an affect on Co levels being released.
So just because you might not detect much or any Co at 1 point in time is no assurance
That as soon as you leave Co levels won't skyrocket.
And if you find a unit to have more than .4" WC on your return a crack is almost guaranteed.

On Lennox clamshell hx's they will crack under the bolt every time
On d amn near any other clamshell hx it will crack at the back where it narrows
The newer Goodman's are NOT using amanas reliable hx and you can expect to find
cracks in those after around 9 years.
Trans will crack right around where the flame is as with most other tube types.
Tempstar eylets get cracks around them.
Armstrongs get cracks alot too

The only hx you will never find a crack in is an oldschool Holland unit.
Unfortunately they went out of business cuz nobody wanted to pay that much for a furnace.

I just looked at a 16 y/o ICP (looks like a Heil maybe) condensing furnace that I know has a problem on the center one of it's 3 burners. I can see the flame change with the blower, but I can't find a crack or a hole in any of the places I can get my borescope into. Any ideas on where these fail. I am just not familiar with them.

I just looked at a 16 y/o ICP (looks like a Heil maybe) condensing furnace that I know has a problem on the center one of it's 3 burners. I can see the flame change with the blower, but I can't find a crack or a hole in any of the places I can get my borescope into. Any ideas on where these fail. I am just not familiar with them.

doesnt mean its a crack, could be its sucking or blowing air from or into the burner compartment

The most common thing that will cause a crack is lack of airflow.
Also Afa Co detectors, they will not sound until they detect... I believe 400ppm.
Also just because you may not pick up any Co means absolutely nothing.
The tiniest most insignificant thing like the ammount of c rap on the burner oriface
Can have an affect on Co levels being released.
So just because you might not detect much or any Co at 1 point in time is no assurance
That as soon as you leave Co levels won't skyrocket.
And if you find a unit to have more than .4" WC on your return a crack is almost guaranteed.

On Lennox clamshell hx's they will crack under the bolt every time
On d amn near any other clamshell hx it will crack at the back where it narrows
The newer Goodman's are NOT using amanas reliable hx and you can expect to find
cracks in those after around 9 years.
Trans will crack right around where the flame is as with most other tube types.
Tempstar eylets get cracks around them.
Armstrongs get cracks alot too

The only hx you will never find a crack in is an oldschool Holland unit.
Unfortunately they went out of business cuz nobody wanted to pay that much for a furnace.

so an AMVC and a GMVC furnaces have different HX? look the same to me....